In bead coating techniques, a bead coating applicator and the moving support on to which the bead is to be coated are in close proximity in a coating zone. Bead formation needs to be controlled if a stable process is to be obtained which permits the use of a wide latitude of coating speeds, layer viscosities and layer thicknesses. Control and stabilization of the bead formation is achieved, first, by using a pressure differential (suction) across the bead at the application locus, and secondly, by applying an electrostatic charge differential just prior to the application locus. Both a pressure differential and an electrostatic charge serve to hold the bead within the coating zone as both these act towards the support aiding the stabilization of the bead and maintaining it in wetting contact with the moving support.
As mentioned above, it is known to use electrostatic fields to improve the uniformity of coatings produced using bead coating techniques. In one known arrangement, a support or backing roller is spaced from a bead coating applicator to form a coating gap therebetween. A high voltage power supply is connected across the backing roller and the bead coating applicator providing a DC voltage of several kilovolts, typically 3 kV, across the coating gap. This DC voltage produces an electrostatic field in the coating gap between the backing roller and the grounded applicator, the backing roller being at a high potential. As a support to be coated is moved through the coating gap, it becomes polarized due to the presence of the electrostatic field thereby producing a given orientation of the dipoles in the moving support. The polarization of the support causes fluid flowing from the applicator into the coating gap to be attracted towards the moving support and to be uniformly deposited thereon.
The actual magnitude and polarity of the electrical potential which needs to be applied to the moving support to generate the polarization thereof is determined by the type of material to be coated, that is, the material of the moving support, and the type of composition to be coated on to the moving support. In some cases, the potential of the coating applicator may be required to be greater or less than ground potential at which it is normally maintained.
However, using voltages of the order of 3 kV or more, as is the case with this arrangement, may create problems with the coating. For example, sparks can be generated making the arrangement unsuitable for use in explosive or volatile environments. In other instances, holes may be produced in the moving support which is to be coated. Furthermore, short circuits or low impedance paths may appear across the coating gap as a result of pinholes existing in the moving support which produces variations in the uniformity of the material being coated.
EP-A-0 055 983 describes an arrangement for applying a bead coating to a moving support which is similar to that described above, namely, that a support or backing roller is spaced from a bead coating applicator to form a coating gap therebetween. However, in this case, the electrostatic charge is not applied to the moving support by an electrostatic field formed in the coating gap. The electrostatic charge is applied to the moving support prior to it reaching the coating gap. This is achieved by generating an electrostatic field on and in the moving support a considerable distance away from the coating gap. The electrostatic field may be generated either using a backing roller and a conductive bristle brush arrangement or using a corona-type arrangement. In both cases, the moving support passes through the electrostatic field produced to receive its electrostatic charge which provides the orientation of the dipoles in the moving support to which the coating material is attracted.
When the backing roller-conductive bristle brush arrangement is used, a relatively intense electrostatic field is established between the free ends of the bristles of the conductive bristle brush and the backing roller with a relatively low voltage. This lower voltage advantageously prevents the occurrence of the problems mentioned above.
Curtain coating techniques differ substantially from bead coating techniques as a freely-falling curtain is formed from a slide hopper which is not in close proximity to the application locus on the moving support. As a result, curtain coating techniques have many advantages over bead coating techniques. In curtain coating techniques, no bead is ever formed and the mechanism of the coating action is distinctly different. For example, the curtain is free-falling and impinges on the moving support with considerable momentum to provide a sufficient force to stabilize the application locus and ensure a uniform wetting line on the moving support. The required momentum is obtained by appropriate selection of the curtain flow rate and the height of free fall.
Other differences are apparent between bead and curtain coating techniques. The effects of coating variables, such as viscosity of the coating composition and flow rate per unit width of coating, are usually completely different in bead and curtain coating techniques.
With bead coming, in order to increase coating speed while retaining coating uniformity, the viscosity of the bottom layer must be reduced thereby increasing the wet coverage of that layer.
However, when curtain coating at high speeds, a high flow rate per unit width can often result in `puddling` of the coating on the support. This commonly occurs when the curtain velocity at the application locus on the support is greater than the velocity of the support being coated. `Puddling` can also occur when the support velocity is greater than the curtain velocity if the momentum of the curtain at the coating application locus is too high. In either case, `puddling` leads to non-uniformities in coating. In contrast to bead coating, these types of coating failures can often be avoided by increasing the viscosity of the coating composition or by lowering the wet coverage of the bottom layer.
EP-B-0 390 774 discloses a method of curtain coating in which it is possible to operate at high coating speeds, with the use of an appropriate level of electrostatic charge, with a particular set of operating parameters such as support smoothness, flow rate, coating composition viscosity and curtain height. The support is moved through the coating zone at a speed of at least 250 cms.sup.-1 and a level of electrostatic charge is applied to the support in accordance with the speed of the support such that the ratio of the magnitude of the charge at any point on the surface of the support to the speed of the support is at least 1:1, the charge being expressed in V and the speed in cms.sup.-1.
EP-A-0 530 752 discloses a coating method in which the phenomenon of air-entrainment is prevented so as to increase the coating speed obtainable during the coating process. The method involves two steps, namely, heating the moving support to a temperature between 35.degree. C. and 45.degree. C. prior to being coated, and applying an electrostatic charge thereto, prior to the application of the coating material. The electrostatic charge can be applied directly to the moving support using a corona discharge electrode or indirectly by applying a high voltage to a backing roller, the backing roller supporting the moving support as the coating is applied. In both cases, the voltages used are in the range of 0.5 to 2 kV.
EP-A-0 563 308 discloses a curtain coating method in which a forward application angle for the freely-falling curtain is utilized to increase the coating speeds obtained. (Application angle is defined as the slope angle of the support at the point of impingement of the freely-falling curtain and a substantially vertical curtain, measured as a declination from the horizontal in the direction of coating.) A freely-falling curtain of the composition to be coated on to a support is directed on to the support as it is moved through a coating zone. The curtain and support are positioned relative to one another so that the curtain impinges on the support in the coating zone with a forward application angle between the curtain and the uncoated support.
Coating speeds in curtain coating are severely limited at high curtain flow rates by the formation of a metastable region. The metastable region is discussed and illustrated in EP-A-0 563 308 and EP-A-0 563 086. It is understood that when curtain coating at moderate to high flow rates, the coating speed at which air-entrainment commences is higher than that at which it clears. At intermediate coating speeds, coating is metastable with respect to any disturbance which may lead to air-entrainment. For practical purposes therefore, these intermediate coating speeds cannot be utilized.
As described in EP-A-0 563 3(5)8, forward application angles in curtain coating allow an increase in the maximum practical coating speed by suppressing the metastable region. The appropriate application angle to give the optimum improvement is dependent on the product being coated, e.g. the wet thickness of the product.
As discussed above, it is well known to use electrostatic charges in curtain coating techniques. This is generally referred to as `polar charge assist`. The effect of `polar charge assist` is to increase the maximum practical coating speed attainable before air-entrainment disrupts the coating. To date it has been understood that significant increases in coating speed are only achievable with reasonably high voltages. However, with voltage levels above about 1200 V, corona discharge at roller nips can fog sensitized photographic products. Moreover, the use of voltages around or above 500 V may also lead to coating defects.
In addition, defects clue to local variations in the electrostatic charge on the support may also result in non-uniform coatings. One of these defects is charge induced mottle.
Another such defect is due to patterns on the surfaces of rollers utilized during the coating process, for example, at the rollers employed at the charging point where the electrostatic charge is applied to the moving support, at the roller over which the support passes at the coating point, and at the face rollers located between the charging point and the coating point. The patterns on the rollers produce a variable gap between the surface of the rollers and the support. This variable gap changes the capacitance of the support, and hence the charge thereon, which causes non-uniform electrostatic fields producing non-uniformities in the resulting coating.